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Physics p1-4 > Topic Two; Electricity > Flashcards

Topic Two; Electricity Flashcards

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1
Q

What is current?

A

Current is the flow of electrical charge.

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2
Q

Electrical current:

A

Electrical charge will only flow round a complete circuit if there is a potential difference.

Therefore a current can only flow if there’s a source of potential difference.

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3
Q

What is the unit of current?

A

Ampere (A)

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4
Q

Current in a single closed loop:

A

In a single closed loop the current has the same value everywhere in the circuit.

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5
Q

What is potential difference? (P.D)

A

Potential difference (or voltage) is the driving force that pushes the charge round.

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6
Q

What is the unit of PD?

A

Voltage so: volt (V)

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7
Q

What is resistance:

A

Resistance is anything that slows the flow down

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8
Q

What is the unit of resistance?

A

Ohm

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9
Q

The current through a component depends on….

A

The potential difference across it and the resistance of the components.

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10
Q

The greater the resistance across a component…

A

The SMALLER the current that flows (for a given potential difference across the component)

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11
Q

The larger the flow of current…

A

The more charge passes around the circuit

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12
Q

Formula to find the total charge

A

Q= I x T

Coulombs(c) = current(A) x Time(s)

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13
Q

Formula linking PS and current

A

Potential difference (v)= current (A) x resistance (ohm)

V=I xR

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14
Q

The resistance of an ohmic conductor:

A. Doesn’t change with the current
B. Does change with the current

+ points about ohmic conductors

A

For some components, as the current through the is changed, the resistance of the component changes aswell

The resistance of ohmic conductors (e.g a wire or a resistor) doesn’t change with the current.

  1. At constant temperature the current flowing through an ohmic conductor is directly proportional to the potential difference across it. (R is constant in V= IR)
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15
Q

Diodes and filament lamps

A

The resistance of some resistors and components DOES change. E.g in a diode or filament lamp.

When an electrical charge flows through a filament lamp, it transfers some energy to the thermal store of the filament, which is designed to heat up. Resistance increases with temperature, so as the current i creases, the filament heats up more and the resistance increases.

For diodes, the resistance depends on the direction of the current. They will only let current flow in one direction but they have a very high resistance if reversed

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16
Q

Ohmic conductor graph p.g 26

A

It’s a straight line at x=y

The current through an ohmic conductor (at a constant temp) is directly proportional to potential difference so you get a straight line.

17
Q

If the temperature of any resistor goes up …

A

The resistance goes up except from a thermistor.

A thermistors resistance drops in hot conditions.

18
Q

Filament lamp graph p.g 26

A

It’s an s shape

As the current increases the temp of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower- hence the curve.

19
Q

Diode graph p.g 26

A

Starts from 0.7 (don’t need to know)

Current will only flow through a diode in one direction, as shown. The diode has a very high resistance in the reverse direction.

20
Q

What is an LDR?

What does it do?

A

LDR= light dependant resistor

It’s a resistor that is dependant on the intensity of light

In bright light, the resistance falls
In darkness, the resistance is highest

Applications: used for automatic night lights
Outdoor lighting and burglar detectors

21
Q

What is a Sensing circuit

A

Sensing circuits are used to turn on or increase the power to components depending on the conditions that they’re in.

22
Q

Sensing circuit used to control a fan:

A

The fixed resistor and the fam will always have the same potential difference across them because they’re connected in parallel.

The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a components resistance, the more of the pd it takes.

So as the room gets hotter, the resistance of the thermistor decreases and takes a smaller share of the pd from the power supply. (This is because In hot conditions, the thermistors resistance drops. )So the pd across the fixed resistor and the fan rises, making the fan go faster.

23
Q

A sensing circuit for a variable resistor

ASK MR SAEED IF THERES A MISTAKE OM THE GREEN BIT END OF SECOND SENTENCE.

A

If you connect a bulb in parallel to an LDR the pd across both the LDR and the bulb will be high When it’s dark and the LDRS resistance is high

The greater the pd across a component the more energy it gets. So a bulb connected across an LDR would get brighter as the room got darker.

24
Q

What are the two types of circuits?

A

Series

Parallel

25
Q

Series circuits:

A

In a series circuit:
1. The different components are connected in a line end to end between the +ve and -ve supply (except voltmeters which are connect in a parallel but they don’t count as a part of the circuit)

  1. If you remove or disconnect one component, the circuit is broken and all stop
26
Q

In a series circuit the potential difference is…

A

Shared

In a series circuit the total pd of the supply is shared between the various components. So all of the pd add up

V total = V1 + V2 + ….

27
Q

In a series circuit the current is….

A

The same everywhere

The same current flows through all components

i1 = i2 = …

The size of the current is determined by the total pd of the cells and the total resistance of the circuit I.e I= V/R

28
Q

In a series circuit the resistance…

A

Adds up

The total resistance of the two components is just the sum of their resistances:
R total = R1 + R2

The pd across each resistor is lower, so the current through each resistor is also lower. As the current is the same everywhere so the total current in the circuit is reduced when a resistor is added because v= IR

29
Q

Cell potential difference..

A

Adds up

There’s a bigger pd when more cells are in series, if they’re all connected the same way.

30
Q

What is a sensing circuit used for?

A

A sensing circuit can be used to turn in or increase the power to components depending on the conditions that they’re in.

Physics p1-4 (1 decks)

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